Bridges come in a variety of forms, each with different types of support. Some are self-supporting, while others rely on cables, towers and girders. Bridges are often aesthetically pleasing, and the most spectacular examples are often historic landmarks. Bridges have been around for thousands of years and serve a vital role in human transportation.
Beam, or girder, bridges consist of two upright piers that support horizontal beams. The beams may be wooden planks or logs, but today it's more common to see them made from steel girders. As the bridge gets longer without support, it becomes weaker. According to the Public Broadcasting Service (PBS), beam bridges are a simple design and are generally have a single span no longer than 250 feet. The Lake Pontchartrain Causeway in Louisiana links together dozens of beam bridges to form the longest continuous series of 24 miles.
The roof of a covered bridge is designed to keep snow and rain off the bridge timbers in winter. While covered bridges are common on the East Coast of the United States, the longest single-span example is in California. Bridgeport Covered Bridge in the Sierra Nevada mountains has a single span of 251 feet. As such, it is a both a State and National Historic Landmark.
Cantilevers are horizontal beams supported on one end. Typically, two cantilevers meet in the middle. Cantilevers are often made from steel girders or concrete. An example is the Forth Rail Bridge which crosses the Firth of Forth in Scotland. The bridge has two main concrete piers but is supported along its length by concrete pillars.
Arch bridges are among the oldest types of bridges. They consist of a supporting abutment at each end from which the bridge arches outwards. The abutments compress the bridge materials and keep it from spreading out, while the curve supports the weight. Stone is a suitable and common material for such bridges, and it was used in Roman viaducts, a form of arch bridge. Today, arch bridges can be made from steel and concrete.
While you might see a rope bridge in a playground or tree house, you can also find them in the Himalayas. Rope bridges are a simple form of suspension bridge in that they consist of one rope for each hand and a third for the feet. Since they are often strung high above gorges, rope bridges are not for the faint of heart.
Suspension bridges consist of two or more towers, or anchorages, from which strong cables are suspended, hence the name. Horizontal cables, suspended from the main cables, support the roadway. One famous suspension bridge is the Golden Gate Bridge. The anchorages are embedded into rock or concrete blocks and evenly distribute the weight they support.